In recent years, soft silicone hydrogel contact lenses become more and more popular because of their high oxygen permeability and comfort. “Soft” contact lenses can conform closely to the shape of the eye, so oxygen cannot easily circumvent the lens. Soft contact lenses must allow oxygen from the surrounding air (i.e., oxygen) to reach the cornea because the cornea does not receive oxygen from the blood supply like other tissue. If sufficient oxygen does not reach the cornea, corneal swelling occurs. Extended periods of oxygen deprivation cause the undesirable growth of blood vessels in the cornea. By having high oxygen permeability, a silicone hydrogel contact lens allows sufficient oxygen permeate through the lens to the cornea and to have minimal adverse effects on corneal health.
However, all commercially available silicone hydrogel contact lenses are produced according to a conventional cast molding technique involving use of disposable plastic molds and a mixture of monomers and/or macromers. There are several disadvantages with such conventional cast-molding technique. For example, a traditional cast-molding manufacturing process must include lens extraction in which unpolymerized monomers must be removed from the lenses by using an organic solvent. Such lens extraction increases the production cost and decreases the production efficiency. In addition, disposable plastic molds inherently have unavoidable dimensional variations, because, during injection-molding of plastic molds, fluctuations in the dimensions of molds can occur as a result of fluctuations in the production process (temperatures, pressures, material properties), and also because the resultant molds may undergo non-uniformly shrinking after the injection molding. These dimensional changes in the mold may lead to fluctuations in the parameters of contact lenses to be produced (peak refractive index, diameter, basic curve, central thickness etc.) and to a low fidelity in duplicating complex lens design.
The above described disadvantages encountered in a conventional cast-molding technique can be overcome by using the so-called Lightstream Technology™ (CIBA Vision), which involves (1) a lens-forming composition being substantially free of monomers and comprising a substantially purified prepolymer with ethylenically-unsaturated groups, (2) reusable molds produced in high precision, and (3) curing under a spatial limitation of actinic radiation (e.g., UV), as described in U.S. Pat. Nos. 5,508,317, 5,583,463, 5,789,464, and 5,849,810. Lenses can be produced at relatively lower cost according to the Lightstream Technology™ to have high consistency and high fidelity to the original lens design.
But, the Lightstream Technology™ has not been applied to make silicone hydrogel contact lenses. One potential issue in the manufacture of silicone hydrogel contact lenses based on Lightstream Technology™ is that the silicone-containing polymerizable materials of a lens formulation are not soluble in water or ophthalmically compatible solvent (non-reactive diluent). As such a non-ophthalmically compatible organic solvent has to be used and a solvent exchange or hydration process has been carried out in the production. Another potential issue is that the silicone-containing components of a lens formulation left behind on the mold surface may not be water soluble and a non-ophthalmically compatible organic solvent, not water, may have to be used to wash the reusable molds. However, use of organic solvents can be costly and is not environmentally friendly. A water-based mold washing system is desirable.
Therefore, there is still a need for a water-processable polymerizable silicone-containing macromers or prepolymers and for washing, with a water-based system, reusable molds for making silicone hydrogel contact lenses according to the Lightstream Technology™. There is also a need for new actinically-crosslinkable prepolymers suitable for making silicone hydrogel contact lenses with desired bulk and surface properties according to the Lightstream Technology™.